The Beech 1900 landing gear is mechanically locked down with a spring-loaded snapring and ball bearings to lock the ball bearings in place. It shouldn't be possible to retract the landing gear on the ground if those snaprings/ball bearings are in place even if the hydraulic pump is pressurized due to the landing gear geometry. Also a squat switch prevents the landing gear handle from being actuated on the ground. I suspect that the culprit was corrosion in the mechanical downlock mechanism.